[Intel-gfx] [PATCH] drm: fix lut value extraction function

[Daniel Vetter]

On Mon, Apr 18, 2016 at 08:39:00PM +0100, Emil Velikov wrote:
> On 18 April 2016 at 16:53, Daniel Vetter <daniel at ffwll.ch> wrote:
> > On Mon, Apr 18, 2016 at 03:40:11PM +0100, Emil Velikov wrote:
> >> On 18 April 2016 at 13:36, Daniel Vetter <daniel at ffwll.ch> wrote:
> >> > On Mon, Apr 18, 2016 at 12:09:51PM +0100, Lionel Landwerlin wrote:
> >> >> Ping?
> >> >>
> >> >> On 22/03/16 14:10, Lionel Landwerlin wrote:
> >> >> >When extracting the value at full precision (16 bits), no need to
> >> >> >round the value.
> >> >> >
> >> >> >This was spotted by Jani when running sparse. Unfortunately this fix
> >> >> >doesn't get rid of the warning.
> >> >
> >> > It sounded like no bug, and the patch itself fails to appease sparse. And
> >> > I didn't check what's upsetting sparse itself, so figured "nothing to do
> >> > here until a real fix shows up".
> >> >
> >> According to the C99 standard a left shift with negative value is
> >> undefined. And we're hitting this case at full precision ;-)
> >
> > Well commit message says sparse is still unhappy. So I'm not sure whether
> > the fix is good enough? And the issue with compiler/static checker noise
> > is that we really should aim to shut them up completely, because broken
> > windows and all that (even if it's sometimes a fallacy, I think it applies
> > here).
> Afaics the fix resolves a real bug and the final solution is bug free
> (although one can drop the L form 1UL). If I have to guess I'd say
> that sparse does not realise that the precision cannot be greater than
> 16.
> 
> Quick and easy check is to add an early bail out (if bit_precision >
> 16 return user_input). The compiler will optimise it out anyway (it
> does propagate/fold the constants) the end binary will be fine.
> Another approach is the earlier suggested, switch which will also get
> optimised in the final binary.

Ok, count me convinced ;-) Applied to drm-misc.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch